1. Field of the Invention
The present invention relates to a new process for the preparation of coatings on any substrates using a coating composition having a two-component binder containing a polyisocyanate component and a special, essentially hydroxyl group-free, isocyanate-reactive component.
2. Description of the Prior Art
Two-component coating compositions which contain as binder a polyisocyanate component in combination with a reactive component capable of reacting with isocyanate groups, particularly a polyhydroxyl component, are known. They are suitable for the preparation of high quality coatings which may be rendered rigid, elastic, resistant to abrasion and to solvents and, above all, resistant to weathering.
The process according to the invention described in more detail below involves the use of new types of two-component coating compositions wherein the binder is based on a two-component system of a polyisocyanate component and an isocyanate-reactive component. This isocyanate-reactive component contains a mixture of compounds having ketimine groups (hereinafter known as polyketimines) with certain secondary polyamines described in more detail below.
The secondary polyamines are the polyaspartic esters described in EP-A-0,403,921. These amines are particularly suitable, in combination with lacquer polyisocyanates, as binders in low-solvent or solvent-free coating compositions and provide rapid curing of the coatings at low temperatures.
The use of polyketimines capable of reacting with polyisocyanates under the influence of moisture for the preparation of polyurethane coatings (the term "polyurethane" also including polyureas) is well known and described, for example, in DE-OS 1,520,139 or DE-OS 3,308,418.
Coating compositions are described in EP-A 0,531,249 which contain as binder a polyisocyanate component which is in combination with an isocyanate-reactive component or which is reactive under the influence of moisture. In one embodiment of this application, the reactive component is a mixture of hydroxy-functional resins with polyaldimines or polyketimines and the above-mentioned polyaspartic esters.
The use of mixtures of polyaspartic esters and polyketimines as the reactive component is not mentioned in EP-A 0,403,921, while according to EP-A 0,531,249 this mixture must also contain hydroxy-functional resins. The reactive component for the polyisocyanate is composed of three completely different individual components, which leads to increased expenditure during the formulation of a lacquer and, also, restricts the mixing ratio between the polyketimines and polyaspartic esters. In addition, it has previously not been possible to prepare hydroxy-functional compounds that have low viscosities similar to polyketimines or polyaspartic esters and yet at the same time exhibit indispensable resin character during curing with low molecular weight, high-functionality polyisocyanates. Therefore, substantial quantities of organic solvents must also be used when such ternary reactive components are employed, which renders the preparation of coating compositions having a high solids content impossible.
It has now been surprisingly found that the use of compounds having hydroxyl groups in combination with polyaspartic esters and polyketimines may be dispensed with altogether in such coating compositions, resulting in compositions which have a high solids content and, after rapid partial drying, exhibit good resistance properties, e.g. resistance to scratching and solvents.